In many medical procedures, various physiological conditions present within a body cavity need to be monitored. These physiological conditions may be physical in their nature, such as pressure, temperature and flow velocity, and may provide the physician or medical technician with critical information as to the status of a patient's condition.
One arrangement that is widely used to monitor physiological conditions in a living body is a sensor wire comprising a sensor element, an electronic unit, a signal transmitting cable connecting the sensor element to the electronic unit, a flexible tube having the sensor element and the cable disposed therein, a solid metal wire (a so-called core wire) having a plurality of sections such that each of the sections has a different flexibility, and a coil that is attached to the distal end of the wire. The sensor element is often present in the form of a microchip, and the signal transmitting cable may be a microcable.
Furthermore, the sensor element may be arranged in a short tube, also referred to as a jacket or a sleeve. The jacket is hollow and accommodates besides the sensor element also a portion of a core wire and often at least one microcable. According to the prior art, the jacket is mainly used to protect the sensor element.
Such an arrangement may be used to determine the ability of a specific coronary vessel to supply blood to the heart muscle, by measuring the intracoronary pressure distally and proximally to a stenosis. The sensor element of such a pressure sensor often comprises a flexible membrane, and the two main types of such pressure sensors are absolute pressure sensors and differential or relative pressure sensors. In an absolute pressure sensor, the membrane is usually mounted across a small cavity wherein a reference pressure, usually vacuum pressure, exists, and the pressure to be measured acts on the opposing surface of the membrane. A differential pressure sensor measures the difference of two pressures acting on opposing sides of the membrane.
The movement or deformation of the membrane can be sensed in different ways by any kind of pressure sensitive element, such as by measuring the changes of electric characteristics of a piezoresistive body, the changes of resistance of an electric conductor or the change of capacitance of a suitably adapted capacitor, which is coupled to the membrane, and which thereby reaches varied forced or strained states as a reaction to any movement of the membrane.
Furthermore, a temperature sensitive resistor may be mounted in the vicinity of the pressure sensitive element, which temperature sensitive resistor has a known temperature dependence, for recording temperature. An electric circuit may also be included, which selectively transfers signals from either of the pressure sensitive element and the temperature sensitive resistor.
Several different designs of sensor wires are known in the prior art, and examples of such sensor wires are described in U.S. Pat. Nos. 6,167,763 B1 and RE35648 E1, disclosing the complete sensor wire, and RE39863, which discloses an arrangement having a “double” Wheatstone bridge. The content of said patent publications is hereby incorporated by reference for the methods and devices disclosed therein.
U.S. Pat. No. 5,113,868 relates to a catheter system including one or several capacitive pressure sensors having a silicon diaphragm. Prior to assembly of the catheter system, the pressure sensors are partially encapsulated with a biomedically compatible material to seal off the hollow cylindrical interiors of the catheter sections from bodily fluids.
U.S. Pat. No. 5,701,905 describes a guide catheter including a sensor element for measurement of blood pressure, and mentions that protective material may be used to maintain the sensor away from the artery walls and to allow the sensor to be directly exposed to blood.
Currently, as described in the above-referenced U.S. Pat. No. 6,167,763 B1, the entire sensor element of the sensor wire assembly is embedded in a soft, elastic material, such as silicone rubber. This protects the sensor element from mechanical impact by surrounding structures while still exposing the membrane to a medium (blood) having the ability to transfer pressure changes, such that the membrane will detect such changes in the fluid passing in the vessel in which the sensor is situated. Further, an embedded sensor is not exposed to blood or other fluids, which potentially could cause short-circuiting in the electric circuits. Thus, the silicone material, e.g. Silicone Dow Corning 734, also functions as an insulator to minimise so-called wet insulation failure.
To comply with The American National Standard for Blood pressure transducers (ANSI/AAMA BP22) each sensor wire is tested for signal stability. Preferably, the assembly must not exhibit more than 5.5 mmHg/h signal deviation under constant conditions. Sensor wires that exceed the above limit should preferably be failed for drift, and this is one of the most common causes of failure during production. Until now, the cause of drift has not been fully understood, and the object of the present invention is to achieve an improved sensor element with regard to signal stability.